M17 MIR: A massive protostar with multiple accretion Outbursts

Abstract

We report the discovery of a massive protostar M17 MIR embedded in a hot molecular core in M17. The multi-wavelength data during 1993 to 2019 show significant mid-IR (MIR) variations, which can be split into three stages, the decreasing phase during 1993.03 to mid 2004, the quiescent phase during mid 2004 to mid 2010, and the re-brightening phase since mid 2010 untill now. The 22 GHz H 2 O maser emission variation, together with the MIR variation, indicate an enhanced disk accretion rate onto M17 MIR during the decreasing and re-brightening phase. Radiative transfer modeling of the spectral energy distributions of M17 MIR in the 2005 (quiescent) and 2017 epoch (accretion outburst) constrains the basic stellar parameters of M17 MIR, which is an intermediate-mass protostar (M * ∼ 5.4 M ⊙ ) with Ṁacc ∼ 1.1 × 10 −5 M ⊙ yr −1 in the 2005 epoch and Ṁacc ∼ 1.7 × 10 −3 M ⊙ yr −1 in the 2017 epoch. The enhanced Ṁacc during outburst induces the luminosity outburst ∆L ≈ 7600 L ⊙ .In the accretion outburst, a larger stellar radius is required to produce Ṁacc consistent with the value estimated from the kinematics of H 2 O masers. M17 MIR shows two accretion outbursts (∆t ∼ 9 − 20 yr) with outburst magnitudes of about 2 mag, separated by a 6 yr quiescent phase. The fraction time of accretion ourbusrt is 83% over 26 yr. The accretion rate in outburst is variable with amplitude much lower than the contrast between quiescent and ourbusrt phase. The extreme youth of M17 MIR suggests that minor accretion bursts are frequent at the earliest stages of massive star formation.